Filter cartridges which are porous, hollow, cylindrical, resin-bonded structures are known in the art for high temperature and/or high strength applications. Particulate matter is entrapped within the pores of the filter media in the cartridge as fluid or gas flows radially inward or radially outward through the media. Various filter media structures have been developed in an attempt to provide an efficient, low-cost and long-lasting filter cartridge. One known filter media structure has a stepped or graded density in the radial direction through the filter cartridge. The density of the filter media increases in the direction of fluid flow (radially inward or radially outward) to trap the larger particulate matter in the more porous (less dense) areas, while the smaller particulate matter is retained in the less porous (more dense) areas. Such filter cartridges are preferable to a single, high density filter cartridge because larger particulate matter contacting the surface of a high density filter may completely plug or fill the small spaces or voids in the media. Such filter media structures are shown in U.S. Pat. Nos. 5,122,270; 3,347,391; 4,661,132; 5,269,921; 3,450,632; 4,240,864; 5,340,479; 4,731,184; 4,629,474; 3,399,516; 4,111,815; and 3,347,391.
The filter media for such cartridges can be formed in many different ways, for example the media can be blown as fibers onto a spinning mandrel, or the media can be formed in a mat, such as by a needling machine. In the latter case, the mat can be wound around the mandrel in a single winding, or built-up in a number of windings. Harwood, et al., U.S. Pat. No. 5,039,413 describes a multi-layer filter cartridge wherein a mat of filter media is wound in a strip around the cartridge such that each individual winding overlaps onto an adjacent winding at least 50% of the width of the wrap. The overlapping of the windings is provided for strengthening the filter cartridge.
It is also known to increase the exposed surface area of a filter cartridge by pleating (see e.g., U.S. Pat. No. 4,731,184) or by cutting into the exterior surface of the filter media so as to form circumferential grooves (see e.g., U.S. Pat. No. 3,347,391). Increasing the exposed surface area of a filter cartridge can increase the useful life of the cartridge because the cartridge can collect and retain a larger amount of large particulate matter. The above techniques of pleating and cutting, however, can require additional manufacturing steps which can increase the overall cost of the filter cartridge, while the technique of cutting can also waste material.
It is therefore believed that there is a demand in the industry for an efficient, low-cost and long-lasting filter cartridge which i) does not require additional manufacturing steps such as cutting or pleating to increase the exposed surface area of the cartridge, and ii) does not waste material during manufacture.